Description of Related Art
An epitaxial bipolar junction transistor (BJT) exhibits the presence of defects at the monocrystalline/polycrystalline boundary of the base. The defects may include misfit dislocations, stacking faults, screw dislocations, and the like. Such defects may cause unacceptable current leakage in the BJT. Where the defects are large enough, a short may even occur between the emitter and the collector.
FIG. 9 illustrates an existing BJT 10. The BJT 10 includes a substrate 12, a collector structure 14 disposed in substrate 12, a buried layer 16, and deep trench isolation (DTI) structures 18. BJT also includes shallow trench isolation (STI) structures that include a source-proximate STI (source STI) 20, an emitter-proximate STI (emitter STI) 22, and a base-proximate STI (base STI) 24. Upon substrate 12, an epitaxial layer is formed that includes a monocrystalline epitaxial base 26 and a polycrystalline epitaxial base 28. An emitter structure 30 is disposed above the epitaxial layer. With the interface between monocrystalline epitaxial base 26 and polycrystalline epitaxial base 28, a leakage region 32 occurs due to crystalline defects and other reasons.
One method of reducing the leakage is to heavily dope the monocrystalline-polycrystalline boundary region with an element that will electrically insulate, in order to enclose the leakage region 32. The implanted, doped enclosure 34 may reduce or significantly eliminate the possibility of the defects being an additional source of leakage in the BJT 10. In order for the implant to get through, a sufficient amount of a first space 36 needs to remain between the edge of the monocrystalline base 26 and the polysilicon of emitter structure 30, where the leakage region 32 may be found. Additional space 38 is needed between the edge of the polysilicon of structure 30 and the emitter cut to avoid the high dose of boron from diffusing from doped enclosure 34 to the emitter/base junction.
The need for both the heavy doping and the spaces 36 and 38 causes the BJT 10 to be large in cell layout size. A large cell layout size increases parasitic capacitance and resistance, both of which are associated with the base and collector. This increase degrades the performance of the BJT 10. FIG. 10 is a top plan view of projected perimeters of a BJT layout 10 formed according to known technique, that may be correlated to FIG. 9. A collector perimeter 40 comprises the overall outline of the projected perimeter of the BJT layout 10. Within collector perimeter 40 is a base perimeter 42, the emitter cut 44 or emitter opening, a collector tap 46, and a base tap 48. Additionally, the doping pattern that fills the first space 36 is seen as a perimeter around the polysilicon of emitter structure 30. In the known technique, the base perimeter 42 may substantially encompass the perimeter of polysilicon that is emitter structure 30, and the projection of base tap 48.